Hydrogen sulfide at high pressure: change in stoichiometry

Hydrogen sulfide (H$_{\mathrm{2}}$S) was studied by x-ray synchrotron diffraction (XRD) and Raman spectroscopy up to 144 GPa at 180-295 K. We find that H$_{\mathrm{2}}$S compound become unstable with respect to formation of new compounds with different composition including pure S, H$_{\mathrm{3}}$S and HS$_{\mathrm{2}}$ depending on the thermodynamic P-T path. These results are supported by our quantum-mechanical variable-composition evolutionary simulations that show the stability of the above mentioned compounds at elevated pressures. The stability of H$_{\mathrm{3}}$S at high pressures, which we find a strong experimental and theoretical confirmation here, suggests that it is this material which is responsible for high-temperature superconducting properties reported previously.